The present invention generally relates to a device for sensing acoustic signals and more particularly relates to a dual beam doppler shift hydrophone wherein pressure variations due to an acoustic wave impinging on a series of mirrors are changed into modulations of light which are transmitted by fiber optic guides.
A majority of the presently used hydrophones are piezo-electric transducers and their use imposes many difficulties. Each such hydrophone requires associated electronic circuitry which distributes power for its operation. There is also a limit as to the minimum of the size and weight of piezoelectric hydrophones. Thus, there is a need for a hydrophone that senses underwater sound with a light beam thereby not requiring telemetry electronics which are sensitive to electromagnetic interference and which can be smaller in size and simpler to operate.
An optically operated hydrophone previously used frequency modulation of a light beam in response to acoustic pressure variations utilizing the doppler effect. This was fully described in my U.S. Pat. No. 4,188,096. Whenever the relative distance between a source of light and a receiver is varying in time, the light received is shifted in frequency in proportion to the relative velocity of the two. A device which provides a change in optical path length with the acoustic pressure wave can thus be employed as a light modulator. The device may include two mirrors between which a light beam propagates through a series of reflections while the distance between the mirrors is varying with pressure variations. It can also be a light transparent block of rubber whose dimensions vary with acoustic pressure variations or simply an optical fiber made of glass or other compressible optical material which changes in length due to applied acoustic pressure variations. The signal thus imposed on the light beam is then demodulated using techniques of interferometry, where the frequency shift of the light beam is detected by comparing it with a second light beam as a reference beam on the surface of a photodetector. Frequency stability, intensity and polarization of both beams are important for proper demodulation and it is thus desirable that the light beams be derived from the same optical source which can be a laser. Furthermore, the path length of both the beams should be matched to be within the coherent length of a laser used. For best performance, both the beams should be exposed to the same environmental conditions in order to balance modulating effects caused by static pressure, strain, etc.